Abstract

To manage and control Phytophthora ramorum successfully, it is important to know the amount of phenotypic variation within a given pathogen population. Because the pathogen has only recently been described, there are few studies on morphological and pathological variation of isolates from the United States. One study has compared growth rate on agar, aggressiveness to Quercus rubra (Eastern American red oak), and developmental stability of European and U.S. isolates. In this study, the U.S. isolates were significantly slower growing, less aggressive, morphologically more unstable, and had a larger range in growth rates than the European isolates (Brasier and others unpublished data; Brasier 2003; Brasier and others, this Proceedings). However, there is no information on fungicide tolerance, spore production or aggressiveness to hosts from the western United States to U.S. isolates. This study examines these phenotypic characters among 12 isolates from a broad range of hosts, locations within California and Oregon, and AFLP genotypes. We examine variation in aggressiveness of isolate populations from California. Additionally, we determined whether measurement of lesions on inoculated coast live oak stems is a good measure of pathogenicity.